Gastric acid secretion under basal conditions and in response to graded balloon distension (50 ml, 100 ml, 150 ml) of the antrum was determined in healthy subjects and in peptic ulcer patients. In patients with a quiescent duodenal ulcer or a gastric ulcer antrum distension produced slight acid responses. The acid response to antrum distension was of quantitative importance only in patients with an active duodenal ulcer, amounting to 35 percent of the maximal acid response to pentagastrin. The healthy subjects did not respond with acid secretion to antrum distension. The pH of the gastric contents in the present series was 1.8-3.0, being lowest in patients with an active duodenal ulcer. Thus, the acid secretory effect of mechanical stimulation of the antrum seems qualitatively different in healthy subjects and ulcer patients, with a marked acid response in patients with an active duodenal ulcer.